Process for variable-pressure operation of a forced-flow vapor generator

ABSTRACT

The enthalpies of the working mediums leaving the respective evaporators in the plant are maintained at a difference of at least 30 kcal/kg when the load approaches or exceeds the critical pressure range. The working medium fed to the downstream heating surfaces is drawn chiefly from the evaporator having the highest enthalpy while the working medium which is recirculated is drawn chiefly from the evaporator having the lowest enthalpy.

United States Patent Sauter 1 May 30, 1972 PROCESS FOR VARIABLE-PRESSURE [56] References Cited VAP R GENERATOR 0 3,205,868 9/1965 Schwarz ..l22/406 S [72] Invento Josef Sauter, g n, Switzerland 3,368,533 2/1968 Knizia .122/406 ST Assignee: sulzer a wintenhur, Switzer- Kmzla S land Primary Examiner-Kenneth W. Sprague [22] Flled: 1970 Attorney-Kenyon & Kenyon Reilly Carr & Chapin [2l] Appl. No.: 95,453

[57] ABSTRACT [30] Foreign Application Priority Data The enthalpies of the working mediums leaving the respective evaporators in the plant are maintained at a difference of at Dec. 12, 1969 Switzerland ..l850l/69 least 30 (cal/kg when the load approaches or exceeds he critical pressure range. The working medium fed to the (g! downstream heating Surfaces is drawn Chiefly from the [58] i 40651. evaporator having the highest enthalpy while-the working medium which is recirculated is drawn chiefly from the evaporator having the lowest enthalpy.

6 Claims, 5 Drawing Figures I,2O l9 J u 1 Patented May 30, 1972 3,665,895

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JOSEF SHUTER Patented May 30, 1972 5 SheetsSheet 5 Fig. 5

PROCESS FOR VARIABLE-PRESSURE OPERATION OF A FORCED-FLOW VAPOR GENERATOR This invention relates to a process for operating a forcedflow vapor generator. More particularly, this invention relates to a process for operating a forced-flow vapor generator at variable pressures and at high outputs close to the critical pressure.

Vapor generators have been known in which parallel flows of working medium are traversed over at least two evaporator tube systems each of which includes a plurality of tube lengths arranged in parallel and which cover a boiler wall over at least part of their length. In those instances when the load has been close to the critical pressure, some of the working medium has been fed back by a circulating means to a place upstream of the boiler wall while the remainder of the working medium has been superheated in heating surfaces downstream of the evaporators.

In those cases where the forced-flow vapor generators have been provided with water separators downstream of the evaporator tube systems, the enthalpy of the working medium remaining and passed on to the downstream heating surfaces after separation of the circulated working medium depends very largely on the load and decreases rapidly as the load rises towards the critical pressure. The calorific requirement therefore shifts to a marked extent from the evaporator heating surfaces to the downstream heating surfaces. In order to meet this calorific requirement, the downstream heating surfaces can be suitably oversized and large quantities of water can be injected underpartial load. Also, the calorific requirement can be met by firing by means of special burners, or by connecting and disconnecting downstream heating surfaces. Such solutions, however, are inconvenient and expensive, since they involve considerable enlargement of the expensive downstream heating surfacesl Accordingly, it is an object of the invention to operate a vapor generator on the variable-pressure principle at high subcritical pressures. It is another object of the invention to operate a vapor generator possibly at supercritical pressures with circulation of the working medium. It is another object of the invention to operate a vapor generator with an inlet enthalpy of the heating surfaces downstream of an evaporator as close as possible to the contact characteristic for a downstream heating surface.

Briefly, the invention provides a process in which a difference of at least 3,0 kcal/kg is produced between the enthalpy of the working medium leaving one tube system and that of the working medium leaving one of the other tube systems of a forced-flow vapor generator when the load of the generator approaches or exceeds the critical pressure range. This is accomplished such that the working medium fed to the downstream heating surfaces is drawn chiefly from the tube system for which the enthalpy of the working medium discharged is highest, and the working medium fed to the circulating means is drawn chiefly from the tube system for which the enthalpy of the working medium discharged is lowest.

These and other objects andadvantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

F IG. l illustrates an enthalpy/pressure diagram of the enthalpy conditions during conventional variable-pressure operation of a forced-flow vapor generator with water separators;

F IG. 2 illustrates a vapor-generating plant which is operable in accordance with the process of the invention;

FIG. 3 illustrates an enthalpy/pressure diagram in accordance with the process of the invention;

F IG. 4 illustrates the variation in the flow rates as a function of the load in accordance with the process of the invention; and

FIG. 5 illustrates a second embodiment of a vapor-generating plant for carrying out the process of the invention.

Referring to FIG. 1, an i-p diagram for water is shown in which the parameters are isotherms, curves of equal humidity,

and the limit curves for the beginning and end of evaporation. in addition, the economizer discharge enthalpy i the evaporator inlet enthalpy 1' the evaporator discharge enthalpy i and the enthalpies i and i, for the water and steam, respectively, obtained in a separator downstream of the evaporator, are shown as a function of the pressure p and load L for a conventional forced-flow vapor generator operated on the variable-pressure principle with circulation of the working medium. It is assumed that the flow rate through the evaporator is always l.5 times the quantity of working medium evaporated in the evaporator at full load. The curve i shows the enthalpy which is to be maintained at theend of the superheater, and which corresponds to a temperature of 540 C. The variation with the load of that portion of the ordinate between the curves i and i which corresponds to the quantity of heat to be transferred in the downstream heating surfaces per kg of working medium, clearly reveals that dihiculties must arise due to the steep fall of curve i close to the critical point, since this fall causes the calorific requirement to be shifted suddenly from the evaporator to the downstream heating surfaces. This shift, as already mentioned, can only be accommodated by very inconvenient means.

Referring to FIG. 2, a vapor (steam) generator, which is operable in accordance with the process of the invention, includes a feed valve 1 1 and a feed-water line 12 through which feed water flows into an economizer heating surface 13 and then flows through an economizer discharge line 14 to a mixer 15. The mixer 15 is connected by a suitable line to.a circulating pump 16 which directs the feed water along branch lines 17, 18 to a first evaporator l and a second evaporator ll. The outlets of the evaporators I, 11 are connected by connecting lines 19, 20, respectively, to respective water separators 21, 22 in order to convey the working medium thereto. Each separator 21, 22 functions to separate water from the working medium and to direct the separated water back to the mixer 15 along return lines 23, 24, while the steam which is separated in the separators 21, 22 flows along connecting lines 25, 26, and a header 27 to downstream heating surfaces 28. These downstream heating surfaces 28 connect over a livesteam line 29 with a valve 30 therein to consuming equipment, such as a turbine 31 in order to direct the steam thereto. The waste steam from the turbine 31 condenses in a condenser 32, and the condensate is fed by a condensate pump 33 through a preheater 34 into a feed-water reservoir 35, from which the condensate is fed back through a high-pressure preheater 37 to the feed-water line 12 by means ofa feed pump 36.

ln accordance with the invention, the variable-pressure process is carried out by making the discharge enthalpy of tube system 11, when the load reaches the critical pressure range, higher than that of tube system i by in this case approximately l00 kcal/kg. This is accomplished by reducing the throughput by means of a valve 39 in the branch line 18. As the water delivered to the evaporator ll is reduced, there is, therefore, a considerable percentage increase in the quantity of water yielded in the separator 21 over that yielded in the separator 22. Accordingly, substantially more water is fed back to the mixer 15 along the line 23 than along the line 24, whereas more steam flows along the branch line 26 than along the branch line 25. In spite of these unequal enthalpy conditions, there is no substantial difference in the tube wall temperatures, since, even in the supercritical range close to the critical point, the specific heat of the steam is very high.

The enthalpy conditions for a vapor generator, as in FIG. 2, operated with a variable-pressure up to 280 (atmospheres absolute atm. abs.) in accordance with the process are illustrated in an i-p diagram in FIG. 3. The curve 1' shows the variation in the enthalpy with the load inthe discharge line 14 from the economizer 13; curve i shows the enthalpy variation in the outlet from the mixer 15; curve i and i the enthalpy variations at the outlets of the two evaporator heating surfaces l and 11 respectively; curves i if the water 3 separators 21, 22. In this embodiment it is assumed that the quantity of working medium delivered by the circulating pump 16 is 1.6 times the quantity of working medium evaporated in the two evaporators I, II under full load.

Under a 40 percent load, the feed-water leaves the economizer at i 262 kcal/kg and is then raised to i, 325 kcal/kg in the mixer by adding working medium of enthalpy i 347 kcal/kg from the separators 21, 22. While the enthalpy of the working medium in the evaporator I then increases by 77 kcal/kg to i 402 kcal/kg, the enthalpy of the working medium flowing through evaporator [1 increases by 1.65 times as much to i 453 kcal/kg. The proportion of steam in the evaporator II is therefore at 0.35, whereas the proportion of steam at the outlet from evaporator l is only x 0.18. In the embodiment described above, these unequal conditions arise because evaporator I is supplied with 100 percent of the total quantity of working medium evaporated and evaporator II, due to restriction at the valve 39, with only 60 percent, whereas the heat is distributed to the evaporators in equal parts. As the load rises, the enthalpies i and i rise, and at a load of approximately 78.5 percent at the outlet from evaporator II the upper limit is reached, that is, no more water is separated in the separator 22, as all of the working medium is drawn ofl in the form of saturated steam. At this loading, the percentage of steam at the inlet of separator 21 is approximately x 0.18, so that approximately 82 percent of the working medium fed into the evaporator I is separated in the form of water and fedback to the mixer 15, whereas 18 percent of the working medium fed to the evaporator l flows on to the consuming equipment 31 in the form of steam, mixed with the steam from evaporator II.

If the load increases to 80 percent, in which case the pressure rises to the critical value, the steam leaving evaporator II is superheated by approximately 5 C. There is, therefore, a temperature difference of approximately 5 C. between the outlets of the evaporators I and II. This difference is not important, and the tubes of the evaporators I and II can still be welded together to form a gas-tight wall without risking undue thermal stressing during operation. The mixing of the two flows of steam produces a new enthalpy i which is only slightly different from the general appearance of enthalpy i, and adapts very closely to an easily produced superheater contact characteristic (relatively increasing heat transfer with increasing load). By using the process according to the invention, therefore, the enthalpy at an 80 percent load can be increased by more than 60 kcal/kg, from i 500 kcal/kg to 1' 564 kcal/kg.

When the load rises from 80 to 100 percent, the enthalpy i of the evaporator II increases slightly, a condition being that the enthalpy of evaporator I remains constant at 500 kcal/kg. Since the glow of steam in evaporator I increases in this range, whereas that of evaporator II remains constant, the mixed enthalpy i drops slightly. The temperature difference between the flow of medium in evaporator II and that in evaporator I rises from approximately 5 C. to approximately 10 C. when the load rises from 80 to 100 percent, but the tube systems I and II can still be welded together without risking undue thermal stressing.-

When the load is between 70 and 78.5 percent, the enthalpy i of the steam in the header 27 is equal to the decrease in saturated-steam enthalpy 1' and 1' The enthalpy then rises to the level already discussed as the load increases to 80 percent. For the i curve in the 70 to 80 percent load range, therefore, there is a substantially triangular downward deviation from the smooth path of an ideal contact characteristic, which is shownby a broken line. The maximum deviation is approximately kcal/kg. Since approximately 250 kcal/kg are absorbed in the downstream heating surfaces under this load, the superheater heating surface 28 must, due to this deviation, be approximately 20 kcal./kg.

However, without the process according to the invention, the superheater heating surface 28 would need to be approximately Due to the invention, therefore, the superheater can be made approximately 15 percent smaller. This triangular deviation can be further reduced or flattened in a manner as described below.

Referring to FIG. 4, the flow rates MI and MI! respectively, which are constant relative to the load in both systems, viz. percent of the total quantity of working medium evaporated under full load in system I and 60 percent of the total quantity of working medium evaporated under full load in system H, are plotted against the load II, or steam pressure p for the evaporator I and for the evaporator II. FIG. 4 also shows how these flow rates divide into the evaporated portion, which flows to the downstream heating surfaces, (hatched) and the non-evaporated or circulated portion (not hatched). If the two hatched areas. are superimposed directly on one another, they meet the straight chain line 3, running from the origin of diagram II to the 40 percent point in diagram I at 100 percent load. This flow-rate diagram shows clearly how evaporation takes place primarily in evaporator ll, whereas in evaporator I the water is primarily brought only to saturatedsteam temperature and then evaporated only to a small extent. It can be clearly seen how the entire throughput of evaporator II is evaporated above a 78.5 percent load, sothat the entire increase in the steam supplied must be provided by evaporator I should the load rise beyond this point. Curve MI follows a curious curved path, due to a flow-rate and thermal balance.

Referring to FIG. 5, wherein like reference characters indicate like parts as above, the power plant can also be constructed with modified water separators, a superheater heating surface suspended in front of the evaporator I, and a means for the introduction of economizer water into the water separator for evaporator I. In addition, various controls are incorporated in the plant for monitoring and controlling the operation of the plant. In this plant, the working medium flows from evaporator 1 into a conventional water separator 42, with a separating chamber at the top and a water-collecting chamber below. The water separator 41 for evaporator II contains only the separating chamber and has an outlet that is connected by a line 43 to the water-collecting chamber of separator 42. This arrangement has the advantage that separator 41 can be made shorter and more cheaply and also that only one water level need be regulated. For example, the water level in the separator 42 is measured by a level gauge which emits a level signal indicative of the water level to a controller 45 on the feed valve 11, so as to regulate the flow of feed water into the line 12.

The steam outlets from the water separators 41, 42 lead into a common connecting line 49 to a wall heating surface 50 which acts as a superheater. This surface 50 is suspended in front of evaporator l, which, with evaporator ll, forms the vertical combustion-chamber tubing (which is welded together to form a fluid-tight wall) for the vapor generator. As a result of the steam flowing through the superheating surface 50, the heat input into evaporator I is reduced, so that the entire fluidtight wall can be lined with evaporator tubes and, therefore, kept at a substantially uniform temperature to ensure uniform thermal expansion. 1

A line 52 leads from the economizer discharge line 14 by way of a control valve 53 to an injection nozzle 54 in the separator 42. This injection system makes it possible to condense steam of enthalpy i by supply water of enthalpy i The quantity of water discharged with enthalpy i is thus increased, and the quantity of water of enthalpy 2' fed along the line 14 into the mixer 15 is reduced, causing an increase in the inlet enthalpy i, for the evaporators l and II. This causes an increase in enthalpies i and i Of particular interest is the increase in enthalpy i in the load range close to the supercritical pressure (70 to 78.5 percent load in the example in FIG. 3), since this shifts the point of complete evaporation in evaporator II into the lower-load range. This means that the above-mentioned triangular deviation of enthalpy i from the ideal contact characteristic can be flattened by means of the injection of water into the separator 42 from the economizer 13.

In order to flatten the triangular deviation, a signal from a temperature-controlling cascade is utilized. For example, the downstream heating surfaces are subdivided into the heating surface 50, which is suspended in front of evaporator I and forms a first superheater, a second superheater 61, and a final superheater 62. The live-steam temperature at the outlet from the final super-heater 62 is measured by a temperature sensor 63 and a corresponding signal is fed by way of a controller 64, to which a set value signal is passed along a line 65, to an injection valve 66. This injection valve 66 is situated in an injection-water line 70, which branches off the feed-water line 12 and leads into the connecting line between the second superheater 61 and final superheater 62 at an injection point 67. In this way, feed water can be injected into the steam flowing between the superheaters 61, 62. In addition, a second temperature sensor 68 is positioned upstream of the injection point 67 and hasan output which leads to a controller 69. An integral-action controller 71, to which the position of the valve 66 is fed as the controlled condition, has an output connected to the controller 69 to feed a set value signal thereto corresponding to the position of the valve 66. The output from controller 69 operates an injection valve 72 situated in a connection line between the feed-water line 12 and an injection point 73, between the first superheater 50 and second superheater 61 in order to inject feed water thereat.

The position of the valve 72 forms the correcting signal for a controller 55, to which, for example, a load-dependent set value signal is fed along a signal line 56. The output from this controller 55 operates the control valve 53 in the injection system connected between the economizer 13 and separator 42. A temperature-measuring means 57 associated with the line 23 in order to measure the temperature therein influences the controller 55 so that the valve 53 is closed if the temperature rises to the limit fed to the input 58 of the controller 55. This ensures that no cavitation can occur in the circulating pump 16.

The various controllers are arranged in such a way that the valve 66 opens when the temperature measured by the sensor 63 rises; the valve 72 opens when the travel of the valve 66 increases or when the temperature measured by the sensor 68 rises; and the valve 53 moves in a closing direction when the travel of the valve 72 increases.

Alternatively the controller 55 may be constructed as a twostep action controller, so as to open the valve 53 when the valve 72 passes a given closing position, that is, is almost completely controlled. The valve 53 can also be operated in dependence on a load sender or the live-steam pressure.

Obviously, the heat absorption of the evaporator tube systems could be made unequal per kg of working medium by other means than unequal supplies to the systems, as explained with reference to FIG. 2, or by screening of one tube system as shown in FIG. 5. For example, the heating surfaces might be of different dimensions, or the heating surfaces might be exposed to different heat inputs, etc., or such means might be combined as desired.

What is claimed is:

l. A process for variable-pressure operation of a forced-flow vapor generator operated at high outputs close to the critical pressure and having at least two evaporator tube systems traversed by parallel flows of working medium and each comprising of plurality of tube lengths arranged in parallel and covering a boiler wall over at least part of the length thereof and heating surfaces downstream of the evaporator tube systems, said process including the steps of drawing some of the working medium from the evaporator tube systems for circulation to a first point upstream of the eva orator tube systems while directing the remainder of the wor mg medium from the evaporator tube systems to the heating surfaces downstream thereof for superheating therein;

producing a difference of at least 30 kcal/kg between the enthalpies of the working medium leaving the evaporator tube systems when the load reaches or exceeds the critical pressure range; and drawing the working medium for superheating chiefly from the evaporator tube system from which the working medium leaves at the highest enthalpy while drawing the working medium for circulation chiefly from the evaporator tube system from which the working medium leaves at the 4 lowest enthalpy.

2. A process as set forth in claim 1 wherein at a load in the pressure range below the limit of the critical pressure, the evaporator tube system with the lowest discharge enthalpy carries water at saturated-steam temperature while the evaporator .tube system with the highest discharge enthalpy carries superheated working medium.

3. A process as set forth in claim I which further comprises the step of passing the working medium of lowest enthalpy to a water separator, separating vapor and water from said working medium for feeding of the vapor to the heating surfaces and feeding of the water to said first point upstream of the evaporator tube systems.

4. A process as set forth in claim 3 wherein at least two evaporator tube systems are each connected to a water separator with said separators being connected in common on the water side to form a single water level in said separators.

5. A process as set forth in claim 3 which further comprises the step of introducing feed water into the flow of working medium at least between the evaporator tube system of lowest enthalpy and the level of water in the water separator to condense saturated vapor in the water separator.

6. A process as set forth in claim 5 which further comprises the step of super-heating at least some of the saturated vapor in a heating surface suspended in front of the evaporator tube system having the lowest discharge enthalpy. 

1. A process for variable-pressure operation of a forced-flow vapor generator operated at high outputs close to the critical pressure and having at least two evaporator tube systems traversed by parallel flows of working medium and each comprising of plurality of tube lengths arranged in parallel and covering a boiler wall over at least part of the length thereof and heating surfaces downstream of the evaporator tube systems, said process including the steps of drawing some of the working medium from the evaporator tube systems for circulation to a first point upstream of the evaporator tube systems while directing the remainder of the working medium from the evaporator tube systems to the heating surfaces downstream thereof for superheating therein; producing a difference of at least 30 kcal/kg between the enthalpies of the working medium leaving the evaporator tube systems when the load reaches or exceeds the critical pressure range; and drawing the working medium for superheating chiefly from the evaporator tube system from which the working medium leaves at the highest enthalpy while drawing the working medium for circulation chiefly from the evaporator tube system from which the working medium leaves at the lowest enthalpy.
 2. A process as set forth in claim 1 wherein at a load in the pressure range below the limit of the critical pressure, the evaporator tube system with the lowest discharge enthalpy carries water at saturated-steam temperature while the evaporator tube system with the highest discharge enthalpy carries superheated working medium.
 3. A process as set forth in claim 1 which further comprises the step of passing the working medium of lowest enthalpy to a water separator, sepaRating vapor and water from said working medium for feeding of the vapor to the heating surfaces and feeding of the water to said first point upstream of the evaporator tube systems.
 4. A process as set forth in claim 3 wherein at least two evaporator tube systems are each connected to a water separator with said separators being connected in common on the water side to form a single water level in said separators.
 5. A process as set forth in claim 3 which further comprises the step of introducing feed water into the flow of working medium at least between the evaporator tube system of lowest enthalpy and the level of water in the water separator to condense saturated vapor in the water separator.
 6. A process as set forth in claim 5 which further comprises the step of superheating at least some of the saturated vapor in a heating surface suspended in front of the evaporator tube system having the lowest discharge enthalpy. 